Due to the technology developments, modern industrial and medical applications use a large quantity of hydrogen as a raw material or other purposes. Hydrogen, however, is a flammable and explosive gas. When the concentration of leakage hydrogen reaches 4.65 vol % or more in air, a hazard of explosion emerges. Therefore, based on the considerations of industrial safety and environmental concern, hydrogen sensors are widely used in factories, laboratories and hospitals in order to accurately monitor the concentration of leakage hydrogen. However, in addition to a large volume and a high production cost, one disadvantage of conventional hydrogen sensors is that most belong to the category of passive elements. The other additional equipment or a conversion circuit is required to perform the analysis or amplification. Therefore, the conventional hydrogen sensors can not become intelligent sensors. As a result, the development of a new and effective hydrogen sensor that is intelligent and of the active type has become an important topic in modern industries.
In recent years, due to the advance of silicon semiconductor technology, much attention has been attracted on the use of a Pd metaloxide-semiconductor (MOS) structure as a semiconductor hydrogen sensor. The reason for using the Pd metal in the hydrogen sensor lies in that Pd has a good catalytic activity and can dissociate the hydrogen molecule adsorbed to the surface into hydrogen atoms. A portion of the hydrogen atoms diffuses through the Pd metal and is adsorbed to the interface between the metal and the oxide layer. These hydrogen atoms, after polarization, cause a change in the Schottky barrier height between the oxide layer and the silicon semiconductor and thus the electrical properties of the device. In the early days, I. Lundstrom proposed a Pd/SiO.sub.2 /Si MOS field effect transistor structure with a Pd gate [Lundstrom, M. S. Shivaraman, and C. Svensson, J. Appl. Phys., 46, 3876 (1975)]. After the hydrogen being adsorbed to the Pd gate, the altered threshold voltage and terminal capacitance are used as the two bases for the detection of hydrogen. However, the use of a three-terminal device to realize the functions of a two-terminal device not only increases the cost, but also has elevated process difficulties. Furthermore, the quality of the oxide layer will also influence the hydrogen detection capability. In addition to the problem of reliability, the quality of an oxide layer becomes unstable due to the growth of the thin oxide layer is contaminated by the ions or the increase of defects. This results in the surface state pinning of Fermi-level of silicon semiconductor. Therefore, Schottky barrier height is less influenced by the polarized hydrogen atoms and subsequently the hydrogen sensitivity is lower. Many researches focus on how to improve such a problem. For example, A. Dutta et al. [A. Dutta, T. K. Chaudhuri, and S. Basu, Materials Science Engineering, B14, 31 (1992)] used zinc oxide (ZnO) and L. Yadava et al. [L. Yadava, R. Dwivedi, and S. K. Srivastava, Solid-St. Electron., 33, 1229 (1990)] used titanium dioxide (TiO.sub.2) to replace the oxide layer of silicon dioxide. On the other hand, the use of a two-terminal type Schottky barrier diode seems to be a more intuitive approach. Without the unstable factors of the oxide layer, the sensitivity of the device to hydrogen has a significant improvement. Therefore, for example, M. C. Steelee et al. [M. C. Steele and B. A. Maciver, Appl. Phys. Lett., 28, 687 (1976)] proposed a Pd/CdS structure, and K. Ito et al. [K. Ito, Surface Sci., 86, 345 (1982)] proposed a Pd/ZnO structure. The using II-VI compound semiconductor as the material is mainly due to the less effect of surface states of II-VI compound semiconductor as compared to the polarized hydrogen atoms.